Manufacturers, transporters, and users of industrial chemicals generally need to track the amount of inventory present in their storage vessels, or to detect varying material levels in process-control systems. A number of instruments designed to be mounted inside the vessels are available for this purpose, including capacitance probes, pressure transducers, and air-ranging ultrasonic transducers. However, the design and shape of the vessel may make application of systems that make contact with the liquid in the vessel impractical, thus making a non-contact system such as the ultrasonic transducer the most cost-effective and preferred approach.
Liquid-ranging ultrasonic transducers designed to mount on the outside of the vessel are available. These units are designed to mount under the bottom of the vessel in such a way that their radiating surface imparts ultrasound energy into the bottom surface of the vessel. The ultrasound energy is then transferred into the liquid contained in the vessel. By recovering energy reflected from the liquid surface, this type of transducer can provide the information needed to determine the liquid level. This approach has not gained widespread use due to a number of limitations. Among these are the requirement that the bottom surface of the vessel be accessible, and of reasonably flat shape. Another requirement is that the vessel be made of metal. Many acid storage vessels are constructed of polymeric materials. These factors do not affect air-ranging transducers.
Air-ranging ultrasonic transducers are typically mounted over openings on top of, or inside the tops of, storage vessels in such a way that their radiating surfaces direct the acoustic sound waves down toward the material surface. Many industrial chemicals, including solvents and acids, give off vapors that condense on the air-ranging transducers subjecting them to chemical degradation.
Various materials have been employed for the construction of the outer components of ultrasonic transducers, primarily the enclosure and the radiating surface, in attempts to make them as chemically resistant as possible. Enclosures made from stainless steel or polymeric materials are typical, as are radiating surfaces made from fluoroplastics such as Polytetrafluoroethylene (PTFE). All of these materials are nevertheless subject to deterioration under prolonged exposure to some chemicals. The polymeric material covering the radiating surface of transducers is typically in the form of a relatively thin membrane. The membrane should be thin in order to preserve the transducer efficiency, enabling the transducer to transfer the maximum amount of acoustic energy into the gaseous medium in which the units operate. The porous nature of the membrane will, in time, allow some chemicals to permeate them, degrading the internal transducer components. These factors limit the applications to which existing air-ranging ultrasonic transducers can be subjected for prolonged periods of use.